The present invention relates to a semiconductor light emitting device and a method of manufacturing such a device, and more specifically to a GaN-based blue-purple semiconductor (to be abbreviated as LD hereinafter) or a GaN-based high luminescence blue-green light emitting diode (to be abbreviated as LED hereinafter).
Recently, GaN-based compound semiconductors such as of GaN, InGaN, AlGaN and InAlGaN are being focused on as materials for blue-purple LD's and high luminescence blue-green light emitting diodes, which are designed to be applied to such applications as high density optical disk systems. In this specification, the suffix indicating the composition of the 3 or 4 element compound semiconductor is omitted unless it is particularly necessary.
In the case where an LD is, for example, used as a light source for an optical disk or the like the shorter its wavelength the greater the size of the focus spot can be reduced. Consequently, the employment of such a short wavelength LD is very effective in achieving a high recording density. However, presently GaN-based LD's, the threshold voltage of operation at room temperature is high as 8V or higher, and further its value is seldom consistent even in samples from the same wafer. Thus, at present, the performance of such an LD is not yet at a satisfactory level for practical use.
Ever since the success in manufacturing p-type GaN (to be called p-GaN hereinafter) by doping with Mg, for the first time, much research has been conducted on various types of lasers of the visible light range from near ultraviolet to blue, with the aim to obtain commercial products. Although a few of these succeeded in the production of laser light emission in a very much limited range, these lasers entail a number of drawbacks still to be solved.
One of the major drawbacks is as follows. That is, in the growth process of a conventional GaN-based compound semiconductor layer, it is inevitable that a great number of defects are produced, and this forestalls a uniform light emission from the active layer.
The cause, for the non-uniformity of the light emission, resides in the fact that the growth temperature for the active and guide layers is in the range 760.degree. C. to 800.degree. C., which is much lower than the growth temperature for the cladding layer adjacent thereto, which is 1100.degree. C., and therefore the growth temperature must be greatly increased at the end of the growth of the active layer and guide layer, that is, before the growth of the cladding layer. When the temperature is thus increased, surface defects, or an interface region containing a great number of defects proliferated from these surface defects, are created.
As a result, an interface region having a high defect density and thus a high resistance as compared to other regions is formed. Therefore, due to the variation in the defect density distribution of the interface region, the current density distribution of the active layer becomes non-uniform, and accordingly the light emitting intensity becomes non-uniform. Such non-uniform light emission causes an increase in the threshold current density and the threshold voltage of the LD operation.
When the growth temperature for the cladding layer is lowered, the creation of such an interface region having a high defect density can be suppressed; however at such a low growth temperature, a p-type cladding layer, which is made of either p-AlGaN or P-GaN having a high doping density and good crystal morphology cannot be obtained. Therefore, when the growth temperature is low, the p-type cladding layer grown on the guide layer has a high resistance and poor crystallinity, thereby greatly increasing the operation voltage of the said LD.
As described above, in conventional GaN-based LD's, the growth of the multilayer structure necessary for forming such a device cannot productively be carried out at a single temperature, and therefore a high resistance region is unfortunately created at a hetero-interface between layers having different growth temperatures. As a result, not only is the series resistance increased but also carriers cannot be uniformly injected into the active layer, and therefore the threshold current and voltage for LD operation are increased. Consequently, an operation voltage much higher than the voltage estimated from the forbidden band width is required.
Thus, in order to realize a high reliability blue-purple LD's and a high luminescence blue-green LED's, both of which are designed for low current and voltage operation, it is important to minimize the resistance of the GaN-based LD or LED and to uniformly and efficiently inject carriers into the active layer.
As described above, in connection with a conventional GaN-based LD or LED, it is very difficult to grow a multilayer structure of a low resistance and a high crystallinity all at a single constant temperature, and therefore, for example, when forming a cladding layer, the temperature condition must be switched at the hetero-interface between the cladding layer and the guide layer. However, when the temperature condition is switched, an interface region having a high defect density is created at the hetero-interface where layers formed at different growth temperatures are formed one on top of the other. As a result, an increase in the resistance and non-uniform carrier injection into the active layer occurs.